Surface-mounted antennas are used for global positioning systems (GPS), local area networks (wireless LAN), etc. with carrier waves in a GHz band. The miniaturization of mobile gears proceed at a dramatic speed, and the surface-mounted antennas are required to be small with reduced height, have good radiation efficiency without directionality, and further be operable in a wide band. However, because conventional surface-mounted antennas have characteristics, which are deteriorated with miniaturization and reduction in height, they are not necessarily satisfactory in achieving sufficient miniaturization and reduction in height.
In general, this type of an antenna is designed to have a radiation electrode whose length corresponds to ¼ of wavelength. This is due to the fact that the antenna exhibits the maximum radiation efficiency at ¼ wavelength, and this requirement is particularly important for mobile gears, which should be able to be operated for as long a period of time as possible by one charge of a battery. It is known that when a radiation electrode is arranged on a dielectric substrate, its effective length is inversely proportional to the square root of a dielectric constant ∈r, which is called a wavelength-reducing effect. With the wavelength-reducing effect, the radiation electrode of the antenna can be made shorter, resulting in the miniaturization and reduction in height of the antenna.
The lower the propagation frequency of the antenna, the smaller the antenna can be made by using a material of a large dielectric constant for the substrate. However, there is actually a limit in the use of high-dielectric constant materials, and only dielectric substrates having dielectric constants ∈r of up to about 4 have been put into practical use, because a higher dielectric constant ∈r than the above level causes the problem of impedance matching. Because input impedance at a current-feeding point is likely to change largely in a surface-mounted antenna having a high dielectric constant, it has become difficult to overcome the problem of impedance matching as the miniaturization.
For instance, as shown in FIG. 21, a surface-mounted antenna described in U.S. Pat. No. 5,867,126 comprises a radiation electrode 92 formed on an upper surface 91 of a substantially rectangular substrate 90 and bent in a substantially L or rectangular U shape, with one end open and the other end grounded, and a current-feeding electrode 94 formed on the upper surface of the substrate 90 with a gap 96 to excite the radiation electrode 92, one end of the current-feeding electrode 94 being connected to a current-feeding wire 99. As shown in FIG. 22, its equivalent circuit is a parallel resonance circuit comprising radiation resistance R and inductance L of the radiation electrode 92, capacitance C formed between the radiation electrode 92 and a ground conductor, and capacitance Ci′ formed between the radiation electrode 92 and the current-feeding electrode 94.
In this antenna, high-frequency electric power from a transmission circuit (not shown) is transmitted to the current-feeding electrode 94 via a current-feeding wire 99 on a circuit board, input to a resonance circuit constituted by the radiation electrode 92 and the ground conductor for parallel resonation, and radiated from the radiation electrode 92 as electromagnetic waves. In order that there is no voltage reflection at a current-feeding point 98, impedance matching should be taken.
Various proposals have been made as impedance-matching means for making the input impedance of the current-feeding electrode 94 viewed from the transmission circuit, namely input impedance at the current-feeding point 98, equal to characteristics impedance of 50 Ω. For instance, in the antenna shown in FIG. 21, the radiation electrode 92 is capacitance-coupled to the current-feeding electrode 94, and capacitance Ci′ is set between the radiation electrode 92 and the current-feeding electrode 94 such that the inductance of the radiation electrode 92 is cancelled as shown in the equivalent circuit of FIG. 22.
However, in the conventional antenna shown in FIG. 21, the current-feeding electrode and the radiation electrode are not directly connected but only capacitance-coupled, without using inductance for impedance matching. Accordingly, if this antenna is made smaller and reduced in height, it cannot have high characteristics easy for impedance matching. In addition, omni-directionality is essentially needed in antennas for GPS, wireless LAN, etc., and improvement in radiation efficiency and gain and the expansion of bandwidth are also needed. These points conventionally have not been fully considered.
When there is impedance mismatching, a new matching circuit is sometimes inserted between a transmission/reception circuit and the antenna. However, the addition of a new matching circuit makes the antenna apparatus larger. With respect to an impedance-matching circuit, JP 2000-286615 A discloses a small antenna comprising a substrate of a laminate structure, and a matching circuit formed between laminate layers. However, this antenna not only has a structure complicated, but also suffers from increase in production cost.
U.S. Pat. No. 6,323,811 discloses an antenna comprising a first radiation electrode (radiation electrode on the side of current feeding) and a second radiation electrode (radiation electrode on the side of no current feeding) on an upper surface of a substrate, in a composite resonance state between the two radiation electrodes, and further comprising an electrode for a matching circuit on a side surface of the substrate. In this antenna, the first radiation electrode (radiation electrode on the side of current feeding) is directly connected to the matching electrode at an impedance matching position, but a current-feeding electrode does not have capacitance. Impedance matching is thus achieved by adjusting only the inductance. The electrode structure having such a matching circuit corresponds to a conventional reverse F antenna, an antenna structure inherently easy for impedance matching.
JP 8-186431 A and JP 11-340726 A disclose impedance matching technologies in a unidirectional antenna having a structure comprising a radiation conductor on an upper surface of a substrate, and a grounding conductor formed on the entire bottom surface of the substrate. However, such antenna is not suitable for applications requiring omni-directionality, such as GPS, wireless LAN, etc. This is clear, for instance, from the fact that this antenna has a structure in which a current-feeding conductor formed on an upper surface of a substrate is surrounded by a radiation conductor, resulting in large capacitance coupling. In addition, because no attention is paid to miniaturization, radiation efficiency, gain and bandwidth, there are problems to be solved to use it for GPS, etc.